Can not use function imagecreatefrompng with png file > 1MB in vBulletin ?

[rapidshare.vn]

This is my code :

<?
require_once('./global.php');
imagecreatefrompng("file.png"); //file.png = 1.4 MB

code . . .
?>

If I remove : require_once('./global.php'); functions imagecreatefrompng working normally;
BUT when I resize file.png < 1MB (not remove the line require_once('./global.php'); ) function imagecreatefrompng working normally, too.

What must I have change in VBB for function imagecreatefrompng working normally ?

Sorry about my bad English !